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We have compared the expression of CD2, CD11a/CD18, CD44, and CD58 on αβ and γδ T cells emigrating from the fetal and postnatal thymus. We report that both γδ and the CD4+ CD8- and CD4-CD8+ subsets of αβ T cells express mature levels of the adhesion molecules CD11a/CD18, CD44, and CD58 upon emigration from the thymus. Whereas CD44 is up-regulated on γδ+ thymocytes prior to export, down-regulation of both CD11a/CD18 and CD58 occurs prior to emigration from the thymus, suggesting that down-regulation of these molecules may be a final maturational step taken by developing γδ T cells before their export from the thymus. In contrast, there is continued up-regulation of CD2 on αβ and γδ T cells upon emigration from the thymus and as they move into the mature peripheral T-cell pool. There was also a marked reduction in the number of CD2+γδ T cells exported during fetal development that was associated with a marked reduction in the percentage of CD22+γδ thymocytes exported. The postthymic maturation of CD2 and the other changes in adhesion-molecule expression appear to be independent of extrinsic antigen, as the same changes were observed in the antigen-free environment of the fetus as in the postnatal lamb, which has been exposed to extrinsic antigen. It thus appears that these changes in adhesion-molecule expression are as a result of the normal maturation pathway from a developing thymocyte to a mature peripheral T cell.